Method of rendering ceramic materials water repellent



Patented Jan. 15, 1952 METHOD OF RENDERING' CERAMIC MATE- RIALS WATERREPELLENT James FranklinHyde, Corning,'N. Y.,assign or to Dow CorningCorporation, Midland, Mich., a

corporation of Michigan 7 No Drawing. Application February 23, 1950,

Serial No. 145,905

2 Claims. (01.11742) This invention relates to the treatment of ceramicmaterials with siloxanes.

In the copendingapplication of James F. Hyde, SerialNo. 734,237, filedMarch 12, 1947 and now abandonedit is shown that ceramic materials arerendered water repellent by treating the surfaces thereof with.diorganosiloxanes. In his copending application, Serial No. 43,752 filedAugust 11, 1948, it is shown that ceramic materials may be renderedwater repellent by treatment with solutions of alkali metal salts ofsiloxanes. Both of the above treatments, however, suifer from thedisadvantage that the water-repellent characteristic of the ceramicmaterials does not persist when they are submergedforprolonged periodsof time in sea water.

In the case of treatment with dimethylsiloxanes, the water repellency,as shown by infinite electrical resistance, is retained for several daysin some instances. However, such treatment suffers from the seriousdisadvantage that in many instances glass insulators which have been sotreated fail after a few. hours in salt water. This erratic behaviorrenders the treatment of ceramic surfaces with diorganosiloxanesuntrustworthy for use in marine applications.

It is an object of this inventionto provide ceramic insulators whichWillretain infinite resistance for unlimited periods of timeuponimmersion in a 3.5 per cent salt solution. Another object is to providea method for preparing saltsta-ble water-repellent ceramics which givesconsistently good results. Another object is to provide insulators whichmay be employed under conditions where they are exposed tosea water.

In accordance with this invention, a ceramic surface is contacted withan aqueous solution of a salt of the formula Rsio (ONa) in which g has avalue from10.9'to 1.5'and'R is an alkyl radical of from 1 to 3 carbonatoms inclusive. The concentration of the salt solution is from 1 to 4per cent by weight of salt based upon the weight of the solution. Thesurface is then dried and heated at a temperature of at least 156 C. forat least five minutes. The ceramic surface is then rinsed with an acid,dried, and contacted with a fluid dimethylpolysiloxane, and the surfaceis thereafter heated to at least 150 C. for at least five minutes.

The siloxane salts employed herein are prepared by reacting silanes ofthe type RSiXs, where 2 X is a hydrolyzable radical, with sodiumhydroxide. Alternatively, the salts may be prepared by reactingpartially condensed monoorgano siloxanes with sodium hydroxide. Theabove methods are well known in the art.

The salts employed herein may respond to the formula RSiO'ONa or to amixture of this salt with salts of the formula RSiO /2 (ONa)2 andRSi(ONa)s. In the event a mixture is employed, the various salts arepresent in such amount that the atomic ratio of sodium to silicon variesfrom 0.9 to 1.5. p

The above defined salts are soluble in water and are best employed as anaqueous solution containing from 1 to 4 per cent by. weight salt. Ifdesired, water-soluble alcohols such as methyl, ethyl, propyl, etc., maybe employed in conjunction'with water in order to hasten solution of thesalt. In the salts employed in this invention, the R ,group is methyl,ethyl, or propyl. When the alkyl radical exceeds three carbon atoms inlength, the tendency of the salt to hydrolyze in aqueous solution is sogreat as to render higher alkyl salts unsuitable for use in thisinvention.

The ceramic surfaces may be contacted with the aqueous salt solution byanyconvenient method. Such methods include dipping, spraying, orpainting. After the surface has been thoroughly Wet with the solution,it is then dried and baked for at least five minutes at a temperature ofat least C. It is to be understood that the time and temperature areminimum and that temperatures ranging up to 450 C. may be employed andtimes up to one hour. No advantage is. obtained by heating the surfacesfor longer than one hour.

It is important that the ceramic surface be thoroughly clean beforeapplication of the salt solution. "One convenient method'for cleaningthe materials is to first boil them in distilled water, followed bygentle boiling in the siloxane salt solution. It has been found that thesiloxane salts not only act as water-repellent agents, but also act ascleaning agents for ceramic surfaces. It has been found thatsatisfactory results are obtained if the ceramic material is immersed inthe boiling siloxane salt solution for a period of fifteen to thirtyminutes.

The salt-treated surface, after drying and heating, is then rinsed withan acid and thereafter washed with water until the surface is free ofelectrolytes. It has been found that any acid which does not attackceramic materials may be employed for this purpose, but the pre- 3fen-ed acids are fatty acids such as acetic, propicnic, etc.

After rinsing with the acid solution, the surface is dried and thencontacted with a liquid methylpolysiloxane. The siloxanes employedherein are essentially free of monomethylsiloxane units but may containsmall amounts of trimethylsiloxane units. In general, the siloxane has adegree of substitution of 2 to 2.1 methyl radicals per silicon atom. Thesiloxane may be applied by any convenient method, such as by dipping,spraying, or painting. In general, it has been found most convenient toemploy the siloxane in the form of a solution having a concentration offrom 1 to 4 per cent siloxane based on the weight of the solution. Anyinert solvent, such as hydrocarbons, ethers, and ketones, may beemployed in this step. Preferably, the solvent should have a boilingpoint below 200 C. in order to facilitate its removal.

After application of the fluid siloxane, the ceramic surface is againbaked at temperatures of at least 150 C. for at least five minutes. Ifdesired, temperatures ranging upwards of 450 C. may be used, and theheating may be continued for one hour or more.

Ceramic surfaces which have been treated in accordance with the abovemethod have an exceedingly high resistance to the effects of sea water.It has been found that the electrical resistance of the surfaces remainsinfinite after a period of one year immersion in a 3.5 per cent sea saltsolution. By contrast, ceramic surfaces which have been treated withsiloxane salts only show poor electrical resistance after a few hoursimmersion, and ceramic surfaces which have been treated with siloxanesonly show poor resistance after a few days under the same conditions.Furthermore, in all cases where the present method has been employed,the surface resistance of the ceramic material has been infinite. Bycontrast, in the case of a large percentage of samples treated with thesiloxane only, the resistance of the samples was very poor after one ortwo days immersion. Thus, the method of this invention not only giveslonger life but more consistently good results than any known method forrendering ceramic insulators water repellent.

The method employed to test the resistivity of the ceramic surfaces isas follows:

The samples were removed from the sea salt water and without any rinsingwere suspended by metal foil bands on spring clips in a humidity cabinetwith the water at 40 C. to give condensing conditions. The surfaceresistance was then measured on a direct current voltage amplifier withthe samples in series with the input and 90 volts applied. The testmethods are more fully set forth by O. K. Johannson and Julius J. Torokin Proceedings of the Institute of Radio Engineers and Waves andElectrons, vol. 34, No. 5, page 298 (May 1946) The following examplesare to be considered as illustrative only of this invention.

Example 1 Ten glass rods, 2 inches long, were each immersed for twentyminutes in a boiling 2 per cent aqueous solution of CHsSiOONa. The rodswere removed and baked ten minutes at 150 C., then rinsed with diluteacetic acid, and finally dried at C. The salt-treated rods were thenimmersed for five minutes in a 2 per cent diethyl ether solution of adimethylpolysiloxane oil having a viscosity of about 1,000 cs. The rodswere then baked thirty minutes at 250 C.

All of the rods were tested by immersing them in a 3% per cent aqueoussolution of sea salt and testing them periodically over a period ofeight months. At the end of this time the rods were removed from thesalt solution and without rinsing were placed in a humidity chamber andtested for electrical resistance as shown above. In all cases the rodsshowed a resistance of greater than 1,000,000 megohms, which is thehighest resistance recorded by the instrument employed.

Example 2 Satisfactory electrical resistance is obtained when glass rodsare treated in the manner of Example 1, employing C2I-I5SiOONa and asthe siloxane salts.

That which is claimed is:

1. A method of rendering ceramic surfaces water repellent whichcomprises contacting the surfaces with an aqueous solution of a siloxanesalt of the formula in which :1; has a value from 0.9 to 1.5, and R isan alkyl radical of from 1 to 3 carbon atoms inclusive, said solutionhaving a concentration of from 1 to 4 per cent by weight of salt basedupon the weight of the solution, drying the surface and baking it at atemperature of at least C. for at least 5 minutes, rinsing the surfacewith a lower fatty acid, drying the surface and contacting it with afluid methylpolysiloxane having a degree of substitution of from 2 to2.1 methyl radicals per silicon atom, and heating the surface at atemperature of at least 150 C. for at least 5 minutes.

.2. The method in accordance with claim 1 in which R is methyl. 7

JAMES FRANKLIN HYDE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,436,304 Johannson Feb. 1'7,1948 2,438,055 Hyde et al. Mar. 16, 1948 2,441,422 Krieble et al May 11,1948 2,507,200 Elliott et al May 9, 1950

1. A METHOD OF RENDERING CERAMIC SURFACES WATER REPELLENT WHICHCOMPRISES CONTACTING THE SURFACES WITH AN AQUEOUS SOLUTION OF A SILOXANESALT OF THE FORMULA